1. Field of the Invention
The present invention relates to an inverter system for feeding AC output to various loads such as a fluorescent light tube for use as the backlight of a liquid crystal display. In particular, the present invention relates to a current detection method of an inverter for detecting change in a circuit current caused by disconnection discharge of circuit wiring of the inverter, proximity discharge between high voltage and low voltage parts of the circuit wiring, and so on, through the medium of magnetic flux change. Further, the present invention relates to a current detection circuit thereof, an anomaly detection method thereof, an anomaly detection circuit thereof, a display device, an electronic device such as a display device and an information processing device, a test method, and a test device.
2. Description of the Related Art
For a liquid crystal display (LCD), a fluorescent light tube (FL tube) is used as a light source for backlight use, and an inverter is used as a lighting device. The inverter adopts a constant current circuit for adjustment of luminance, and so on, and the FL tube is driven by a constant current to be lit up with a low current of about several mA at a high voltage, for example, in the order of 1.5 kV. A current route including the FL tube to which AC output from the inverter is supplied corresponds to a segment of wiring, leading from a winding of an inverter transformer, on the high voltage side thereof, to the FL tube, and is susceptible to deformation under external pressure because it is long and thin in size since it passes by way of a multitude of members such as windings of the inverter transformer, a conductor pattern on a printed circuit board, connectors, wiring, etc. With a cellular phone and a note type personal computer, an installation space for the FL tube lighting device is particularly narrow, so that thin wiring leading from the inverter to the FL tube is susceptible to deformation due to external pressure.
In case that disconnection occurs to the above-described current route of the inverter, flow of current is cut off by the disconnection, however, there arises a risk of the current route being maintained because the constant current at the high voltage flows in the current route of the FL tube, and a rise in voltage occurs at the time of the disconnection, thereby resulting in continuation of discharge at a spot of the disconnection. When the current route is maintained, current flows in the tube, so that the FL tube remains lit up. Since it is impossible to know behavioral anomaly from a lighting state, discovery of that behavioral anomaly is delayed. Continuation of such an anomalous state is not preferable.
Therefore, concerning an inverter for feeding AC output to various loads such as a fluorescent light tube for use as the backlight of a liquid crystal display, technical development which relates to a device for avoiding the continuation of anomaly and displaying such anomaly by detecting anomaly of disconnection discharge of circuit wiring, dielectric breakdown discharge between high voltage and low voltage parts and ground-fault discharge is desired.
Concerning a technology relating to detection of behavioral anomaly of an inverter and avoidance of continuation of the behavioral anomaly, as earlier patent documents, the Japanese Patent Laid Open Publications No. 6-140173, No. 11-121190, No. 9-113546 and so on are in existence.
According to the Publication No. 6-140173, as a protecting device for a discharge lamp lighting system, even if an discharge lamp is lit out at the time of anomaly, inconvenience to maintain the operation of a transistor inverter is avoided wherein an output of the inverter is taken out to monitor an oscillation condition by a monitor circuit, and anomaly occurring at the oscillation condition due to a short circuit inside the discharge lamp and so forth is detected by a monitoring pulse voltage so as to stop the operation of the inverter by stopping the feed of a DC voltage, and light out the discharge lamp. However, such a protection device is complex in configuration because it requires the monitor circuit for taking out the output of the inverter and monitoring thereof, a microcomputer for determining whether the monitoring pulse voltage is normal or anomalous, and so forth, and also since the output of the inverter is taken out to an outside, the inverter side is forced to take measures to cope with the change in an operating condition.
Further, according to the Publication No. 11-121190, as a discharge lamp lighting device, noting that a high frequency voltage value is lowered when the high frequency voltage to be fed to an discharge lamp is discharged between a ground and a low voltage part, the high frequency voltage is detected by a high frequency voltage detection resistor connected to the discharge lamp, and the high frequency voltage is rectified to be converted into a DC voltage, and when a level of the DC voltage is in anomalous level, the discharge lamp lighting device is stopped in operation. However, the discharge lamp lighting device requires the high frequency voltage detection resistor directly connected to the discharge lamp, and also the difference in voltage at the time of normal and that at the time of anomaly (at the time of discharge) is determined by a voltage dividing ratio of a resistance voltage divider circuit configured by the high frequency voltage detection resistor and a threshold level of a switching transistor, and so forth, and hence the accuracy of determination of whether it is normal or anomalous is low, and also there is a risk of stoppage of operation by a level change at the time of normal. Even with the discharge lamp lighting device, since high frequency voltage detection resistor is connected to the high voltage part side to detect a high frequency voltage, special measures for changing a circuit condition and taking out the detection voltage are required.
Furthermore, the Publication No. 9-113546 relates to an overcurrent detecting circuit, and discloses that a current transformer detecting a current fed to a load from a switching power source section is used. The overcurrent detecting circuit controls a switching power source section by an overcurrent detection signal which is caused based on a detection of an overcurrent fed to the load, and makes DC output voltage droop from the switching power source section to the load. This overcurrent detecting device is not a device which detects change in a circuit current due to discharge occurring a current route or a load side.
Meanwhile, if there occurs discharge at a spot of the break of circuit wiring and between high voltage and low voltage parts, an electric change due to that discharge can be checked. When discharge occurs to a disconnection spot, a voltage value or current value representing discharge is increased inside the inverter, but the amount of change is small, and hence even if the detection voltage is simply compared with a reference voltage, when the difference in level therebetween at the time of anomaly and normal is small, an erroneous operation is easy to occur, and hence it is not practical. In order to enhance accuracy of the detection of disconnection discharge or dielectric breakdown discharge between high and low voltage parts, a circuit having the combination of a differentiation circuit, a timer circuit and so forth is required, which however renders the circuit complex, which increases in manufacturing cost by adding a circuit like this to the inverter, resulting in lack of practice. In the case where the amount of change in waveforms is small at the time of normal behavior and at the time of discharge such as a very small discharge, sufficient accuracy of detection cannot be obtained even if a differentiation circuit is used, so that an erroneous operation is easy to occur and it lacks in reliability.
In the case of driving a load such as an FL tube by using a inverter, it is indispensable for maintenance of reliability of operation of an inverter and a load side thereof to discover disconnection discharge or discharge between high voltage and low voltage parts occurring in the current route and to prevent the continuation of an anomalous state. However, these problems are not disclosed in the Publications No. 6-140173, No. 11-121190 and No. 9-113546, and means for solving the problems is not presented either.
Further, an obstacle due to very small discharge of an LCD is a defect due to assembly work of the LCD. Although that defect is detected by a visual confirmation and so on, certainty and reliability of that detection are lower. Many of ground-fault discharge occurring between a high-voltage part and the ground around thereof are due to damage occurring in a covering of a high-voltage wiring, engagement between parts of wiring, a tear of an insulating tube of a soldered part, and so on, and is also due to an impression of stress to semiconductors in the LCD. Because of this, an additional test of dielectric strength is required. Further, disconnection discharge of a high-voltage current route is due to disconnection of a soldered part, contact fault of a connector, disconnection of a wiring, and so on. Although an electric detection method of discharge like this is performed by observation of current waveform, preparation of a testing equipment and time of a test are longer. Further, for an inspection, a visual confirmation of an appearance is also performed. Even a product to which such a test has been performed, a detection of a fuming obstacle occurring after the lapse of time is difficult. Therefore, in the LCD, a test method and a test device which can discover an obstacle such as disconnection discharge and ground-fault discharge with ease and can obtain a test result with high reliability are desired.